super precision bearings for machine tools factors … · for optimal performance, machine tools...
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SUPER PRECISION BEARINGS
FOR MACHINE TOOLS
FACTORS FOR A SUCCESSFUL
SPINDLE BEARING SET UP
P:800.225.4587 E: [email protected] www. emersonbearing.com Machine Tool Spindle Bearing Selection and Mounting Guide 2
Why Angular Contact Bearings? Angular Contact bearings are suited for high
speed applications where both axial and radial loads exist and rigidity is
required thru preloading. They are typically used in pairs.
Optimal bearing selection must be determined thru considerations about the
properties required by the main spindle being used, in terms of: maximum
speed, radial and axial rigidities, main spindle size, required accuracies,
and lubrication systems. An understanding of the following is necessary to
choose the optimum bearing for the application.DID
YOU
KN
OW?
P:800.225.4587 E: [email protected] www. emersonbearing.com Machine Tool Spindle Bearing Selection and Mounting Guide 3
WHAT TO KNOW FOR A SUCCESSFUL SPINDLE BEARING SET UP?
Rigidity of Spindle
Preloading
Elimination of all radial and end play
Increased system rigidity
Limiting change in the contact angle between the inner
and outer ring at high speed
Prevention of ball skidding under high acceleration
(courtesy Barden)
System rigidity in machine tool applications is extremely important because the
magnitude of deflection under load determines machining accuracy. Bearing rigidity
is only one factor that influences system rigidity; others include shaft diameter, tool
overhang, housing rigidity and type of bearings used.
For axial loading applications angular contact ball bearings are normally used
as their larger contact angle provides higher axial rigidity. If high radial rigidity
is needed cylindrical roller bearings are preferred as they provide more surface
contact although they are not suitable for very high-speed applications.
This is the removal of internal clearance by placing a thrust load on the bearings.
Giving preload to a bearing results in the rolling element and raceway surfaces to
be under constant elastic compressive force at their contact points. This has the
effect of making the bearing extremely rigid so that even under load, radial or axial
displacement does not occur. There are typically four increasing Preloads available
Light, Normal, Medium or Heavy. It is important to choose the correct preload for
your needs.
Preloading
is used to
satisfy one of
the following
requirements:
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Duplexed (paired) bearings
control preload by having a
preload offset which is a function
of relieving of the faces of the
inner and outer rings. When
the bearings are clamped into
position the offset faces are
brought together introducing
a preload.
Back to Back duplex (DB). The wider distance between the effective load centers of the DB arrangement allows for larger moment loads to be handled with rigidity. They can sustain radial and axial load in both directions.
Face to Face duplex (DF). This has a shorter distance between the effective load centers which reduces capacity for handling moment loads however it is preferred in slower speed
applications where shaft binding due to misalignment may
occur. They can sustain radial and axial loads in both directions.
Tandem duplex (DT). In tandem duplex arrangements the preload offsets of both the inner and outer rings create parallel lines of contact resulting in capacity to handle both
large radial and large axial loads, but only in one direction. Many main spindles of machining centers use four row (DBBT) arrangements offering high rigidity and high speed.
Based on a users’
specifications angular contact
bearings can
be arranged
in various
combinations.
Bearing Arrangement Options
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Contact Angle Options
The combination of radial play and thrust loading creates a line of contact between
the balls and the raceways.
For applications where extreme rigidity is required these are the solution with the
same envelope dimensions as standard precision bearings.
Ball Screw Support Bearings
Provide high rigidity with low drag torque.
Greater control of axial runout.
Higher running speeds and longer life.
Contact angles
can be as high
as 60 degrees.
Advantages:
The standard contact angle is 15-degrees as it provides
optimum combination of axial and radial capacity and rigidity.
The 25-degree contact angle is used where greater axial rigidity
is required. For higher rigidity requirements Ball Screw Support bearings may be used.
This is the
Contact Angle.
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PROPER SPINDLE BEARING CARE AND HANDLING
High Speeds. When the operating speed of rolling bearings exceeds the rated limiting speed wear can render the bearing
unserviceable. The increased sliding speed, stressing and uneven running can lead to failure.
Rapid change of speed. Rapid speed changes can cause high acceleration which can lead to a large degree of slip between
rolling elements and raceways.
Lubricant Service life. Typically, it is assumed that the selected lubricant satisfies the requirements of the bearing and it’s operating conditions-load, speed and temperature. It is often forgotten that, like bearings, lubricants have a defined service life. Lubricity decreases due to aging and mechanical stress, resulting in increased wear. In grease applications for Angular Contact bearings the typical fill is 25% of bearing free space.
Sealing efficiency. There can be different wear cycles for different styles of seals. An example being full contact seals can have greater sealing element wear and maintenance costs.
Environmental Conditions. One of the major reasons for wear is the ingress of dirt and foreign particles. Water and other substances can cause corrosion of the active surfaces.
Issues to
consider:
Bearing Life
Service Life
When bearings rotate the inner and outer rings and the rolling elements are
constantly loaded. This produces material fatigue and eventual failure. The total
number of revolutions before failure occurs is referred to as Basic Rating Life.
While all bearings have an expected service life it is important to avoid issues that
may increase wear and decrease that life expectation.
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Basic Dynamic Load Rating
ABMA Grades of Precision
C2 Clearance Less
Than Normal
C0 Normal Clearance
C3 Clearance greater
than Normal
C4 Clearance greater
Than C3
Radial Internal
Clearance for
Radial Ball
Bearings
The basic dynamic load rating of a bearing with rotating inner ring and stationary
outer ring is that load of constant magnitude and size which a large group of
identical bearings can endure for a basic rating life of one million revolutions.
Ball Bearings are manufactured to precision grades ABEC 1,3,5,7,9. Most standard
bearings for Motors, Pumps and general machine applications are ABEC 1-3. Most
Machine Tool applications require ABEC class of 7-9. The equivalent DIN standards
are P0, P6, P5, P4, P3, P2.
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Bearing Handling
Proper handling of precision bearings is essential to achieving maximum speed and
limited temperature rise.
The cleaning process involves removing the rust preventative oil
that precision bearings are delivered in.
After drying the bearing apply grease aiming for the inner ring surface between balls in equal amounts. For roller bearings, apply grease to the outer or inside of rollers while turning the
rollers to spread the grease.
Running In. For oil lubrication the running-in operation is relatively simple because no peak temperature occurs and the
bearing temperature stabilizes within a relatively short time. For grease lubricated bearings the running in process is more
challenging. During the initial running in a large temperature rise occurs while the bearing speed is increased. It is important to increase the speed incrementally and allow for temperature
stabilization between speed increases.
This involves
cleaning, drying,
filling with grease, and
the running in
operation.
P:800.225.4587 E: [email protected] www. emersonbearing.com Machine Tool Spindle Bearing Selection and Mounting Guide 9
MACHINE TOOL BEARINGS FROM EMERSON BEARING
For optimal performance, machine tools depend on robust bearings that are
specially engineered for high-speed and high-precision machine tool spindles. At
Emerson Bearing, we understand the importance of quality and accuracy when
it comes to the bearings that will be supporting your most sensitive machine tool
applications. Our specialists work closely with customers to identify and supply
the bearing that best suits their unique application.
Emerson Bearing’s extensive product selection includes the newest generation
of bearings made from advanced steels. These bearings are recommended for
sophisticated machine centers with high-speed spindles and strict precision
requirements. We also carry a selection of sealed bearings that are specially
designed to withstand harsh operating environments. All of our bearings come
from trusted, industry-leading brands including Barden, NSK, EZO, and more.
To learn more about our quality bearing solutions and other capabilities,
please reach out to us today.
For over five decades, the Emerson name has developed and maintained
a reputation for high-quality ball and roller bearings. Together, our two
companies—Emerson Bearing and Action Bearing—serve the bearing needs
of international and domestic customers. The team at Emerson Bearing caters
to global niche markets, while the team at our sister company, Action Bearing,
specializes in solutions for the OEM and MRO markets in New England.
We carry several brands of Machine Tool bearings that represent many
countries of origin and several price points and availability. These include:
Barden, EZO, GMN, Nachi, NSK, NTN, RHP and TPI.
ABOUT
EMERSON BEARING
Location:201 Brighton Avenue
Boston, MA 02134
Contact:[email protected]
Visit:https://www.emersonbearing.com/
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